1. Field of the Invention
The present invention relates to method and apparatus of modulating a series of data words into (d,k) constrained sequence with good suppression of a direct current (DC) component.
2. Description of the Related Art
Run length limited codes, generically designated as (d,k) codes, have been widely and successfully applied in modern magnetic and optical recording systems. Such codes, and means for implementing said codes, are described by K. A. Schouhamer Immink in the book entitled “Codes for Mass Data Storage Systems” (ISBN 90-74249-23-X, 1999).
Run length limited codes are extensions of earlier non return to zero recording (NRZ) codes, where binarily recorded “zeros” are represented by no (magnetic flux) change in the recording medium, while binary “ones” are represented by transitions from one direction of recorded flux to the opposite direction.
In a (d,k) code, the above recording rules are maintained with the additional constraints that at least d “zeros” are recorded between successive data “ones”, and no more than k “zeros” are recorded between successive data “ones”. The first constraint arises to obviate intersymbol interference occurring due to pulse crowding of the reproduced transitions when a series of “ones” are contiguously recorded. The second constraint arises in recovering a clock from the reproduced data by “locking” a phase locked loop to the reproduced transitions. If there is too long an unbroken string of contiguous “zeros” with no interspersed “ones”, the clock regenerating phase-locked-loop will fall out of synchronism.
In, for example, a (1,7) code there is at least one ‘zero’ between recorded “ones”, and there are no more than seven recorded contiguous “zeros” between recorded “ones”. The series of encoded bits is converted, via a modulo-2 integration operation, to a corresponding modulated signal formed by bit cells having a high or low signal value, a ‘one’ bit being represented in the modulated signal by a change from a high to a low signal value or vice versa. A ‘zero’ bit is represented by the lack of change of the modulated signal.
The minimum inversion period Tmin, which can be expressed by (d+1)T is thus equal to 2T where T is a bit time interval in the recording wave train. The maximum inversion period Tmax, which can be expressed by (k+1)T, is thus equal to 8T.
By the way, in a train of channel bits generated by a (1,7) code the minimum inversion period Tmin is more frequently observed than inversion periods of length 3T, 4T, etc. The fact that a lot of edge information is generated at short intervals such as 2T and 3T is advantageous to the generation of a clock signal in many cases.
As the recording density is increased, however, the minimum inversion period Tmin this time becomes a problem. That is if minimum runs 2T are generated consecutively the recording wave train is prone to distortion generated therein. This is because a 2T wave output amplitude is smaller than other wave output amplitudes and, hence, easily affected by factors such as a defocus and a tangential tilt.
In addition, at a high line density, recording of consecutive minimum marks (2T) is also easily affected by disturbances such as noise. Thus, an operation to play back the data will also be prone to errors. In this case, a pattern of errors in reproduction of the data is observed as shifts of the front and rear edges of a minimum mark in many cases. As a result, the length of the generated bit error increases.
As described above, when data is transmitted through a transmission line or recorded onto a medium, the data is modulated into a coded sequence matching the transmission line or recording medium prior to the transmission or recording. If the coded sequence resulting from the modulation contains a direct current (DC) component, a variety of error signals such as tracking errors generated in control of a servo of the disk drive become prone to variations or jitter are generated easily.
The first reason for using said dc-free signals is that recording channels are not normally responsive to low-frequency components. The suppression of low-frequency components in the signal is also highly advantageous when the signal is read from an optical record carrier on which the signal is recorded in the track, because then continuous tracking control undisturbed by the recorded signal is possible.
A good suppression of the low-frequency components leads to improved tracking with less disturbing audible noise. For this reason it is thus desirable to make as many efforts to prevent the modulated sequence from containing a direct current component as possible.
In order to prevent the modulated sequence from containing a direct current component, control of a DSV (Digital Sum Value) to prevent the modulated signal from containing a direct current component has been proposed. The DSV is a total found by adding up the values of a train of bits, wherein the values +1 and −1 are assigned to ‘1’ and ‘0’ in the train respectively, which results after NRZI modulation of a train of channel bits. The DSV is an indicator of a direct current component contained in a train of sequences.
A substantially constant running digital sum value (DSV) means that the frequency spectrum of the signal does not comprise frequency components in the low frequency area. Note that DSV control is normally not applied to a sequence generated by a standard (d,k) code. DSV control for such standard (d,k) codes is accomplished by calculating a DSV of a train of encoded bits after the modulation for a predetermined period of time and inserting a predetermined number of DSV control bits into the train of encoded bits. In order to improve the code efficiency it is desirable to reduce the number of DSV control bits to a smallest possible value.
Preferably, the encoded signal comprises a sequence of q code words, where q is an integer. Between encoded signal portions are inserted synchronization (sync) signals. Preferably, the sync signal should not occur in a sequence of the encoded signal. Conventionally, the sync pattern contains a series of s consecutive bits equal to the logical “0”, where s is an integer exceeding k, or alternatively the sync pattern consists of two series of k bits having a logical “0” separated by a bit having a logical “1”, i.e. two consecutive runs of k “0”s.
A disadvantage of the usage of such sync patterns is that they are relatively long, and therefore reduce the efficiency of the recording. Therefore, preferably, a short sync pattern is used, which may comprise a sequence of two or more consecutive “0” runs.
An example of the use of such signals to record and read an audio signal on an optical or magneto-optical record carrier can be found in U.S. Pat. No. 4,501,000. The specification describes the Eight-to-Fourteen (EFM) modulation system, which is used for recording information on Compact Disks (CD) or MiniDisk (MD). The EFM-modulated signal is obtained by converting a series of 8-bit information words into a series of 14-bit code words, and where 3-bit merging words are inserted between consecutive code words.
Respective code words of 14 bits satisfy the conditions that at least d=2 and at most k=10 “0”s are placed between two “1”s. In order to satisfy this condition also between code words, 3-bit merging words are used. Four 3-bit merging words of 8 possible 3-bit merging words are permitted to be used, namely “001”, “010”, “000”, and “100”. The remaining possible 3-bit merging words, namely “111”, “011”, “101”, and “110” are not used as they violate the prescribed d=2 constraint.
One of the four allowed merging words is selected such that the bit string obtained after cascading alternate code words and merging words satisfies the (d,k)-constraint, and that in the corresponding modulo-2 integrated signal the DSV remains substantially constant. By deciding the merging words according to above rules, low-frequency components of the modulated signal can be reduced.
The choice for the 3-bit merging words is based on the requirement that, on the one hand the channel signal is satisfied. Decoding of EFM signals is very simple. A sync pattern is multiplexed between 33 alternative 3-bit merging words and 14-bit code words. The 27-bit sync pattern used in the CD format consists of two consecutive runs of then “0”s plus 3-bit merging word.
The choice of the merging words avoids the occurrence of said sync pattern in the output sequence. The relative occurrence of the sync pattern in this format is, 27 bits in a total of 588 bits, 4.6%. The decoded skips the 3-bit merging words, and the 14-bit code words are translated, using a look-up table or PLA etc., into the information bytes (8 bits).
Information recording has a constant need for increasing the reading and writing speed. The aim of increased reading speed, however, requires higher servo bandwidth of the tracking mechanism, which, in turn, sets more severe restrictions on the suppression of the low-frequency components in the recorded signal.
Improved suppression of the low-frequency components is also advantageous for suppressing audible noise arising from the tracking mechanism. For this reason, it is desirable to make as many efforts to prevent the signal from containing low-frequency components.